Ceramics are widely used in many fields thanks to its excellent mechanical properties such as heat resistance and abrasion resistance, as well as electric and magnetic properties and biocompatibility. Among them, ceramic sheets including zirconia as a main component can be effectively used as sensor parts, electrolyte film for solid oxide fuel cells and setters for calcination because of its excellent oxygen ion conductivity and heat and corrosion resistance.
Usually, the above-described ceramic sheet including zirconia is produced by the following method. First, a slurry containing zirconia powder, organic binder, and a solvent is formed into a sheet by a doctor blade method, a calendar rolling method or an extrusion method. The resultant sheet is dried to evaporate the solvent to form a green sheet. The green sheet is arranged to a suitable size by cutting or punching, and then placed on a setter and calcined to decompose or remove the organic binder and to sinter the ceramic powder.
As to the zirconia powder used as raw material for ceramics formed products, there are various reports on its production methods and the physical properties of the ceramics produced by using the zirconia powder. However, most of them refer to only a particle diameter of the zirconia powder, but are silent regarding the particle size distribution. In fact, few of them refer to both the particle diameter and the particle size distribution.
Japanese Unexamined Patent Publication No. 1-153530 describes a ceramics formed product produced by using zirconia powder having a primary particle diameter of 0.1 to 0.5 .mu.m, and the particles of 90 volume percent of the zirconia powder preferably have a diameter of 0.1 to 1 .mu.m. Japanese Unexamined Patent Publication No. 4-130018 describes a ceramics formed product produced by using zirconia powder having an average particle diameter of 1.3 to 3.0 .mu.m measured by centrifugation, in which particles having a particle diameter of 1 to 20 .mu.m accounts for 45 to 75 weight percent of the entire zirconia powder. Japanese Unexamined Patent Publication No. 4-202016 describes three types of zirconia powder, that is: (1) a zirconia powder having a particle diameter of 0.60 to 4.00 .mu.m and an average particle diameter of 2.05 to 2.12 .mu.m, in which the particles having a particle diameter of 1.00 to 3.00 .mu.m accounts for 87 to 90 percent of the entire zirconia powder; (2) a zirconia powder having a particle diameter of 0.80 to 4.00 .mu.m and an average particle diameter of 2.18 to 2.22 .mu.m, in which the particles having a particle diameter of 1.00 to 3.00 .mu.m accounts for 82 to 85 percent of the entire zirconia powder; and (3) a zirconia powder having a particle diameter of 0.88 to 4.00 .mu.m and an average particle diameter of 2.00 to 2.04 .mu.m, in which the particles having a particle diameter of 1.00 to 3.00 .mu.m accounts for 86 to 90 percent of the entire zirconia powder.
However, the ceramic sheets produced by using the above-described conventional zirconia powders are likely to have warping and waviness. Such ceramic sheets do not have a flat surface, and have poor load resistance and bending strength. These problems become especially serious in producing a ceramic sheet having a large size and thin thickness.
In order to solve the problems of the prior art, the present inventors have proposed a novel ceramic sheet and a method for producing the same in Japanese Unexamined Patent Publications Nos. 8-151270, 8-151271, and 8-151275. In Japanese Unexamined Patent Publications Nos. 8-151270, 8-151271, and 8-151275, a ceramic sheet is produced using a ceramics powder having an average particle diameter of 0.1 to 0.5 .mu.m, in which the particles of 90 volume percent of the ceramic powder have a diameter of 1 .mu.m or smaller. The ceramic powder is produced by the following method. First, a raw material powder having an average particle diameter of 1.5 .mu.m, in which the particles of 90 volume percent of the powder have a diameter of 3 .mu.m or smaller is mixed with water to prepare a slurry containing 20 weight percent of the raw material powder. The slurry is milled with a bead mill for 2 hours, thereby obtaining the ceramic powder.
However, this method has a problem. As described above, the slurry contains 20 weight percent of raw material powder and it takes two hours for milling the slurry into the ceramic powder. This results in low productivity. In order to increase the productivity, it may be considered that larger amount of raw material powder is used in the slurry. However, the mere increase in the amount of raw material powder used simply extends the milling time, sometime the slurry becomes too viscous to mill any more and no improvement of productivity will result.
Furthermore, there is another problem as follows. In general, it is preferable that the ceramic powder has fine particles with narrow particle size distribution, i.e., the standard deviation of the distribution curve is small. When the ceramic sheet is produced from a green sheet including a ceramic powder having fine particle size, for example, an average particle diameter of 0.1 to 0.5 .mu.m, in which the particles of 90 volume percent of the ceramics powder have a diameter of 1 .mu.m or smaller, large amount of binder is required to produce the green sheet. When the green sheet includes large amount of binder, the binder cannot be sufficiently removed when the green sheet is fired. This results in the formation of warping or waviness in the resultant ceramic sheet, or non-uniformity of the mechanical strength on the surface of the ceramic sheet.